To identify previously unrecognized sources for acquiring acute hepatitis B and non-A, non-B (NANB) hepatitis, we interviewed patients with these types of hepatitis who were reported to two county health departments in the United States and matched control subjects for known and potential risk factors for acquiring hepatitis. Of 218 patients with hepatitis B and 140 patients with NANB hepatitis, 46% and 53%, respectively, had no commonly recognized source for infection. When these patients were compared with control subjects, significantly more patients with hepatitis B had multiple heterosexual partners, accounting for 14% of all hepatitis B infections; more patients with NANB hepatitis either had sexual or household contact with a person who had hepatitis in the past or had multiple heterosexual partners, accounting for 11% of all NANB infections. This is the first study to suggest that heterosexual transmission may play an important role in the spread of NANB hepatitis.